Alfalfa (Medicago sativa) is an important and valuable forage and feed crop throughout the world. Alfalfa exhibits traits setting it apart from many other crop plants. It is an auto-tetraploid and is frequently self-incompatible in breeding. When selfed, little or no seed is produced, or the seed may not germinate, or when it does, it may later stop growing. Typically, fewer than five percent of selfed crosses produce seed. When a very small population is crossbred, inbreeding depression occurs, and traits of interest, such as quality, yield, and resistance to a large number of pests (e.g., seven or eight different pests), are lost. Thus, producing a true breeding parent for hybrids is not possible, which complicates breeding substantially.
Some sources indicate that there are nine major germplasm sources of alfalfa: M. falcata, Ladak, M. varia, Turkistan, Flemish, Chilean, Peruvian, Indian, and African. Tissue culture of explant source tissue, such as mature cotyledons and hypocotyls, demonstrates the regeneration frequency of genotypes in most cultivars is only about 10 percent. Seitz-Kris, M. H. and E. T. Bingham, In vitro Cellular and Developmental Biology 24 (10):1047-1052 (1988). Efforts have been underway to improve regeneration of alfalfa plants from callus tissue. E. T. Bingham, et. al., Crop Science 15:719-721 (1975). Some methods for regeneration of alfalfa plants from tissue culture are described in U.S. Pat. No. 5,324,646 issued Jun. 28, 1994, which is hereby incorporated by reference.
Additionally, researchers believe that somatic embryogenesis in alfalfa is inheritable, and is controlled by relatively few genes. Efforts at improving regeneration have thus been directed towards isolation of the genetic control of embryogenesis, and breeding programs which would incorporate such information. See, e.g., M. M. Hernandez-Fernandez, and B. R. Christie, Genome 32:318-321 (1989); I. M. Ray and E. T. Bingham, Crop Science 29:1545-1548 (1989).